In broadband communication systems, a number of individual channels are often multiplexed for transmission over a common medium in order to save transmission cost. One method of multiplexing makes use of wavelength (or frequency) division, were individual channels are carried over different non-overlapping spectral bands. With such a method a communication system typically includes the following components between a signal source and a destination:
a) a multiplexer for grouping more than one individual channel into a signal form suitable for transmission over a single medium, PA1 b) a transmission medium, PA1 c) a demultiplexer for extracting individual channels from the multiplexed signal.
In addition, a variety of other system components are interspersed among the above components, such as amplifiers, modulators, demodulators, filters, etc. Many of such components introduce some degree of non-linearity in the signal leading to undesirable distortion and errors. It is normally desirable to maintain a flat spectral response within each channel in the system so that different wavelengths undergo similar gain or attenuation when passing through various stages of a communication system.
A variety of spectrum equalizing (or flattening) techniques have been developed to address such problem. For example, in U.S. Pat. Nos. 5,532,870 and 5,640,269, Shigematsu et al disclose an optical fiber amplifier which reduces the wavelength dependency of gain in various wavelength ranges in wavelength division multiplexing transmission, by using at least two kinds of optical fibers serially coupled each having a glass composition selected from at least two kinds of rare-earth-doped glass compositions. Another example is the optical amplifier disclosed by Minelly and Laming in U.S. Pat. No. 5,526,175, which amplifies signals of different wavelengths throughout a spectral window while equalizing the output levels of the signals, by using a dichroic reflector at one end of an amplifying fiber to set up standing wave patterns therein by interference of the forward and reflected signal lights, at the different wavelengths. Furthermore, daSilva et al disclose in U.S. Pat. No. 5,345,332 a technique for channel-by-channel power regulation in a multiwavelength lightwave communications system, by using a cascade of inhomogeneously broadened saturated fiber amplifiers spaced along the optical fiber transmission path.
The above disclosed techniques, however, are either too complex or more applicable to band-limited optical communication systems and may, therefore, present expensive solutions for systems transmitting a relatively large number of multiplexed channels. Such a problem is of a particular concern in optical communication systems where the selection of equalizing filters is more limited than in traditional radio communication systems. There is, therefore, clearly an important need for more economical solutions for equalizing wavelength-division multiplexed channels, especially in the case of lightwave communications.